Absorbers are widely used to remove substances such as gases and particulate matter from combustion or flue gases produced by utility and industrial plants. Often of particular concern is sulfur dioxide (SO.sub.2) and other acidic gases produced by the combustion of fossil fuels and various industrial operations. Such gases are known to be hazardous to the environment, and therefore their emission into the atmosphere is closely regulated by clean air statutes. Generally, these acidic gases are removed with spray towers or other types of gas-liquid contactors through the use of wet flue gas desulfurization (FGD) processes.
The cleansing action provided by gas-liquid contactors is generally derived from the passage of flue gases upwardly through a tower countercurrently to a descending liquid that cleans the air. Wet flue gas desulfurization processes typically involve the use of an alkaline cleansing liquid, such as a calcium-based slurry or a sodium-based or ammonia-based solution. As used herein, a slurry is a mixture of solids and liquids in which the content of the solids can be any desired level, including the extreme condition in which the slurry is termed a moist solid. Examples of calcium-based slurries are limestone (calcium carbonate; CaCO.sub.3) slurries and hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Intimate contact between the alkaline liquid and acidic gases present in the flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the acidic gases by the slurry. Thereafter, the liquid is accumulated in a tank where the absorbed acidic gases are reacted to form precipitates that can be collected for disposal or recycling. For example, in a flue gas desulfurization process using a calcium-based slurry, the byproduct precipitate is gypsum (CaSO.sub.4).
A known type of absorber 10 of the type using a spray tower 14 as a gas-liquid contactor is shown in FIG. 1. The spray tower 14 is generally an upright structure equipped with an inlet duct 12 through which flue gases enter the absorber 10. The inlet duct 12, as well as other appropriate sections of the tower 14, are preferably formed from a high nickel alloy to promote their corrosion resistance. Above the inlet duct 12 are banks 16 of spray headers 18 which introduce a spray of cleansing liquid, such as a calcium-based alkaline slurry, into the tower 14. Any number of banks 16 and spray headers 18 can be used as may be required for a given application. One or more pumps 26 are required to recycle the slurry to the spray headers 18 from a reservoir or tank 22 in which the slurry accumulates after contact with the flue gases. Each bank 16 of spray headers 18 may be individually equipped with a pump 26 to promote the flexibility of the pumping and spraying operation to accommodate varying demands by the scrubbing operation. After being "scrubbed," the flue gases are permitted to escape to the atmosphere through a mist eliminator 24 at an upper end of the tower 14.
Intimate contact between the slurry spray and the flue gases rising through the tower 14 results in the acidic gases being absorbed by the slurry, which is then collected at the bottom of the tower 14 in the tank 22. As indicated in FIG. 1, the tank 22 conventionally requires an aerator 28 and one or more agitators 30. The aerator 28 injects an oxygen-containing gas, such as air, into the slurry accumulated in the tank 22 so that the slurry reacts with the absorbed acidic gases to form solid precipitates, such as gypsum if a calcium-based slurry is used, that can be safely recycled or disposed. The agitators 30 are required to continuously mix the slurry in order to maintain the alkali and solid precipitates in suspension. As shown, the tank 22 is equipped with an overflow 20 to limit the level of slurry in the tank 22, and is adapted to receive additional alkali 32 to compensate for that which has reacted with the acidic gases.
As is typical, the agitators 30 are shown as fans. Though this type of agitator is known to be effective, the gas bubbles generated by the aerator 28 are distributed throughout the slurry and lower its density, causing the slurry to occupy a considerable portion of the tower 14. From a structural standpoint, expansion of the slurry is disadvantageous because it necessitates that the height of the tower 14 be significantly greater than would be otherwise necessary to accommodate a suitable quantity of slurry. From an operational standpoint, the high speed of the fan blade tips causes secondary nucleation of solids, resulting in finer precipitates that are more difficult to remove from the tank 22 and more difficult to dewater and dry after removal. Further disadvantages with the use of fan agitators 30 are the occurrence of pump cavitation due to the intake of bubbles through the pump inlet 40 in the tank 22, and the energy and costs to operate and maintain the agitators 30.
Those skilled in the art will appreciate that, in view of the considerations noted above, it would be desirable to reduce construction, operational and maintenance costs that are attributable to the agitation of a slurry within the tank of a flue gas absorber.